Riser stabilisation

ABSTRACT

A method of stabilizing a riser includes applying an oscillating force to the riser, using one or more thrust units on the rise, so as to counteract oscillating movement of the rise, preferably having a first component oscillating at a period between 3 and 20 seconds and a second component oscillating at a period between 1 and 5 minutes, where the thrust units may be positioned at antinodes of the oscillating movement and preferably each comprise three pairs of thrusters respectively configured to provide thrust in three evenly spaced directions.

TECHNICAL FIELD

The present invention relates to stabilizing an offshore riser.

BACKGROUND

An offshore riser is a conduit that connects a subsea oil well, or thelike, on the seabed to a surface platform. There are a number of typesof risers including export and production risers, drilling risers andworkover risers. Fatigue of risers is a common challenge for many typesof risers. A combination of platform motion, wave forces and vortexinduced forces from currents acting on the riser induce fluctuatingtension and bending moments in the riser. The fluctuating bending momentinduced stresses especially can cause fatigue damage to the riserresulting in a short fatigue life.

When a riser reaches a certain level of fatigue damage, it must beinspected and sometimes replaced (depending on the safety standard,inspection method, detected cracks etc). Inspection and especiallyreplacement of the riser joints is an expensive process, with a riseritself often costing as much as $100 million. Furthermore, maintenanceand replacement of the riser incurs further costs associated with lostoperation/production time during the maintenance or replacement. It istherefore highly desirable to maximize the operational life of the riserby increasing its fatigue life, which is often a limiting factor.

One known problem for risers is vortex-induced vibration (VIV). Thistype of vibration occurs in elongate bodies interacting with an externalfluid flow and can be responsible for fatigue damage to risers. VIVarises when a boundary layer of the fluid separates from the bodycausing vortices to form in the wake of the body, changing the pressuredistribution along the rear surface. Due to irregularities in the flow,the vortices do not form symmetrically around the body and fluctuatinglift forces develop on each side of the body, thus leading to relativelyhigh-frequency oscillatory motion transverse to the flow direction.

A large number of solutions have been previously proposed to address theproblem of fatigue caused by VIV by suppressing the formation ofvortices about the riser. This is typically achieved using flow guides,such as helical strakes or fairings formed along the length of theriser.

In addition to reducing fatigue damage by reducing VIV, the operationallife of a riser may also be increased by reducing the magnitude of thestatic forces to which it is subjected. WO 01/71153 A1 suggests a methodof reducing bending forces arising at the lower portion of the riserthat arise from static current forces acting along the length of theriser. WO 01/71153 A1 uses a number of propeller-driven thrusterspositioned along the length of a riser to apply constant forces inpredetermined directions that counteract static ocean current forces inorder to maintain a desired riser course.

Thus, various devices have been proposed to address VIV and damage fromstatic forces. However, other problems remain.

BRIEF SUMMARY

Viewed from a first aspect, the present invention provides a method ofstabilizing a riser connecting a structure on the seabed, such as awellhead, to the sea surface, the method comprising: applying anoscillating force to the riser using a thrust unit on the riser, so asto counteract an oscillating movement of the riser.

The inventors have realized that there is a significant problem offatigue caused by oscillating motion of a riser. This has not beenaddressed in the prior art. The use of a thrust unit to provide anoscillating force on the riser allows for the damaging oscillatingmovement of the riser to be actively cancelled. By providing an activethrust to counteract the oscillations of the riser the oscillations ofthe riser can be negated to a far more significant degree than ispossible using passive countermeasures.

In this context, a thrust unit is intended to include any device capableto producing an active force on the riser by ejection of mass, whichshould preferably be a device that can generate sufficient thrust tocause movement of the riser. For example, the thrust unit may compriseat least one propeller driven thruster or at least one water jetthruster. A propeller driven thruster is a thruster which generatesthrust by moving water immediately adjacent to the thruster through apropeller. A water jet thruster is a thruster which ejects high-speedwater that is stored in or collected from a remote location andtransported by pipe to the thruster location.

It should be understood that the thrust unit may be mounted to theriser, for example being formed separately and mounted in accordancewith the following method, or the thrust unit may be formed integrallywith the riser, for example it could be formed together with one of theriser segments during manufacture.

The oscillating force from the thrust unit is applied at a locationalong the length of the riser. The precise location will vary dependingon a number of factors, such as the particular riser properties, localweather conditions, number of thrust units used, etc., but the locationis typically between 10% and 90% along the length of the riser.

Furthermore, as used herein the terms oscillating force and oscillatingmovement are intended to refer to a force and a displacement,respectively, having a repetitive variation in time. For example, theoscillation may be a repeated sinusoidal type variation in time.

It has been identified that the most damaging oscillations occur atperiods much longer than those typically observed in VIV. Therefore,whilst the method can be applied to vortex induced vibrations, theoscillating movement of the riser to be counteracted preferably has aperiod of over 1 second.

The oscillation also preferably has a period of less than 30 minutes. Inpreferred embodiments, the oscillating motion has a period of between 3seconds and 20 seconds, and more preferably between 6 seconds and 15seconds, which correspond to typical periods for ocean waves. In otherpreferred embodiments the oscillating motion has a period of between 1minute and 5 minutes, which corresponds to typical periods of slow driftoscillation of a surface vessel caused by wind and wave drift forces.

Furthermore, oscillations at resonant frequencies of the riser areamplified by this resonance making them particularly damaging; typicallythe lowest resonant frequency has the highest degree of amplification.Thus, preferably the oscillating movement of the riser to becounteracted is at a resonant frequency of the riser, and morepreferably is at the lowest resonant frequency of the riser.

The oscillating movement is often not a single-mode vibration, butrather includes components oscillating at a number of different modes.The oscillating force may therefore also be applied to the riser so asto further counteract oscillating movement of the riser at one or moreadditional, different frequency, preferably also at a resonant frequencyof the riser. For example, the oscillating force may be to counteractoscillating movement of the riser having one component with a period ofbetween 3 seconds and 20 seconds (preferably between 6 and 15 seconds)and another component with a period of between 1 minute and 5 minutes.

The method may comprise the step of locating the thrust unit on theriser, for example by mounting the thrust unit on the riser. Preferablythe method comprises locating the thrust unit on the riser at apredetermined location, this location being selected to maximize thecorrective effect of the thrust unit on expected oscillating movementsof the riser.

The oscillating movement will typically form a standing wave having atleast two nodes (points where the wave has zero amplitude) and at leastone antinode (points where the amplitude of the standing wave is amaximum). It is desirable therefore that the thrust unit is located onthe riser about at an antinode of the oscillating movement of the riserbecause the thrust generated by the thrust unit will provide the maximumcancelling effect when applied at the antinode of the standing wave.

The method may therefore further comprise: modelling the oscillatingmovement of the riser; predicting the location of an antinode of theoscillating movement based on the modelling; and locating the thrustunit on the riser at the predicted antinode location. The method mayinclude predicting the locations of multiple antinodes and locatingmultiple thrust units on the riser at two or more of the predictedantinode locations.

Typically, at the frequencies of interest, the ends of riser may beconsidered to be nodes of the riser, as they remain relatively stillcompared with mid-points of the riser. Therefore, the thrust unit is notlocated at an upper end or a lower end of the riser, and preferably notwithin 10% of the length of the riser from the upper or lower ends ofthe riser.

In a further aspect therefore, the present invention may also be seen toprovide a method of locating a thrust unit on a riser, the methodcomprising: modelling oscillating movement of the riser; predicting thelocation of an antinode of the oscillating movement based on themodelling; and locating the thrust unit on the riser at the predictedantinode.

A further oscillating force may be applied to the riser using a secondthrust unit configured to be located on the riser at a location axiallyoffset from the first thrust unit. As mentioned above, the second thrustunit is preferably also positioned at an antinode of the oscillatingmovement of the riser. However, it may of course be positionedelsewhere. The antinode made be an antinode of a different mode ofvibration of the oscillating movement of the riser from that of thefirst thruster.

The or each thrust unit preferably comprises a pair of thrusters onopposite sides of the riser, wherein first and second thrusters of thepair may be aligned and hence provide thrust in parallel with eachother. By using a pair of thrusters on opposing sides of the riser, thepair of thrusters can generate an uninterrupted flow (i.e. notinterrupted by the riser itself) without inadvertently generating atorsion force about the riser, which could cause damage to the riser.

More preferably, the or each thrust unit comprises at least two pairs ofthrusters with the pairs being arranged to provide thrust innon-parallel directions, and even more preferably the thrust unitcomprises at least three pairs of thrusters, each pair being arranged toprovide thrust in a direction non-parallel with the direction of thrustsof the other pairs.

It is preferable that the or each thrust unit be able to generate a netthrust in any horizontal direction. In order to achieve this, forexample, either a single thruster must be able to rotate or at least twothrusters must be able to generate thrust in two non-paralleldirections. Generating thrust in non-parallel directions is preferableto a rotatable thruster as the force applied can be adjusted morerapidly in response when a change in thrust direction is required. Thegeneration of thrust in at least three non-parallel directions(preferably three evenly-spaced directions) is preferred because thisprovides redundancy. With the use of three non-parallel directions thethrust unit can still operate even if thrust in one direction isunavailable, for example if a thruster is damaged or is providinginsufficient thrust, because only thrust in two non-parallel directionsis required to generate a net thrust in any horizontal direction.

In one embodiment, the pairs of thrusters are axially offset from oneanother so that their respective output streams do not significantlyinterfere with one another. In the present context, the term axially hasbeen used in reference to the axial direction of the riser.

In an alternative embodiment, a first thruster of each pair of thrustersis located in a first thrust plane and a second thruster of each pair ofthrusters is located in a second thrust plane, which is axially offsetalong the axis of the riser. The thrust planes are preferablysubstantially horizontal, or perpendicular to the axis of the riser.

This embodiment results in a more compact configuration. Furthermore,this configuration avoids axial bending moments from being induced bythe thrust unit because the center of thrust for each pair of thrustersis the same.

The first thrusters in the first thrust plane and the second thrustersin the second thrust plane may have the same configuration rotated by180°. By this configuration, torsion generated in one thrust plane iscancelled by an equal and opposing torsion generated in the other thrustplane.

Viewed from a second aspect, the present invention provides a riserstabilization system for stabilizing a riser connecting a structure onthe seabed, such as a wellhead, to the sea surface comprising: at leastone sensor for detecting movement of the riser; a thrust unit forapplying a force to the riser; and a controller configured to, in use,cause the thrust unit to apply an oscillating force to the riser so asto counteract an oscillating movement of the riser detected by the atleast one sensor.

The controller is preferably configured to, in use, cause the thrustunit to perform the method of the first aspect, and optionally any ofits preferred features.

The oscillating movement of the riser to be counteracted preferably hasa period of over 1 second. The oscillation also preferably has a periodof less than 30 minutes. In preferred embodiments, the oscillatingmotion has a period of between 3 seconds and 1 minute, and mostpreferably between 6 seconds and 15 seconds. In other preferredembodiments, the oscillating motion has a period of between 1 minute and5 minutes.

Preferably the oscillating movement of the riser to be counteracted isat a resonant frequency of the riser, and more preferably is at thelowest resonant frequent of the riser. The oscillating force may also beapplied to the riser so as to further counteract oscillating movement ofthe riser at a second, different frequency, which may be a secondresonant frequency of the riser.

The thrust unit may comprise at least one propeller driven thruster orat least one water jet thruster. The thrust unit preferably comprises apair of thrusters mounted on opposite sides of the riser.

The thrust unit may be configured to be mounted to the riser, or thethrust unit may be formed integrally with the riser or a segment of theriser.

It is preferable that the thrust unit be able to generate a net thrustforce in any horizontal direction. Thus, the thrust unit may comprise atleast two pairs of thrusters arranged to provide thrust in non-paralleldirections, and more preferably at least three pairs of thrusters, eachbeing arranged to provide thrust in a non-parallel direction.

In one embodiment, the pairs of thrusters are axially offset from oneanother so that their respective output streams do not significantlyinterfere with one another. In an alternative embodiment, a firstthruster of each pair of thrusters is mounted in a first thrust planeand a second thruster of each pair of thrusters is mounted in a second,axially offset, thrust plane. The thrust planes are preferablysubstantially horizontal, or perpendicular to the axis of the riser.

The system may further comprise a second thrust unit, optionallyincluding any of the preferred features of the first thrust unit. Thus,the oscillating force may be a force applied using two or optionallymore than two thrust units at the same time. The controller may thenfurther be configured to, in use, control each thrust unit independentlyto apply an oscillating force to the riser so as to counteract theoscillating movement of the riser detected by the at least one sensor.

The sensor is preferably arranged to detect at least a horizontalvelocity of the riser. The sensor may be an accelerometer.

Viewed from a third aspect, the present invention further provides athrust unit for mounting to a riser, the thrust unit comprising: a frameconfigured to be mounted to the riser; and a plurality of pairs ofthrusters, each being mounted to the frame, wherein each pair ofthrusters generates a net thrust in a respectively non-paralleldirection.

Preferably, for each pair of thrusters, a first thruster of the pair ofthrusters is configured so as to be on an opposite side of the riser toa second thruster of the pair of thrusters, when the frame is mounted tothe riser. Preferably the plurality of pairs of thrusters comprisesthree pairs of thrusters.

In one preferred embodiment, the pairs of thrusters are mounted to theframe so that, when mounted to the riser, each of the pairs of thrustersare axially offset from the other pairs of thrusters.

In another preferred embodiment, a first thruster of each pair ofthrusters is mounted in a first thrust plane and a second thruster ofeach pair of thrusters is mounted in a second thrust plane that isaxially offset along the axis of the riser, wherein the thrusters ineach of the first and second thrust planes are configured to generate anet thrust in any direction within the thrust plane.

The invention extends to a riser for connecting a structure on theseabed, such as a wellhead, to the sea surface, comprising a system or athrust unit as described above, wherein the thrust unit is mounted onthe riser. The thrust unit is preferably mounted on the riser at aboutthe location of an antinode of a mode of oscillating movement of theriser.

The thrust unit mounted at a location along the length of the riser,i.e. the thrust unit is not located at an upper end or a lower end ofthe riser, and preferably between 10% and 90% along the length of theriser.

Viewed from a fourth aspect, the present invention provides a computerprogram product comprising computer readable instructions that, whenexecuted, will cause a processor to perform a method comprising:receiving indications of a movement of a riser connecting a structure onthe seabed, such as a wellhead, to the sea surface from at least onesensor; and causing a thrust unit to apply an oscillating force to theriser so as to counteract oscillating movement of the riser detected bythe at least one sensor.

The computer program product preferably comprises computer readableinstructions that, when executed, will cause a processor to perform themethod performed by the controller in the first and second aspects, andoptionally any of the preferred aspects.

Preferably, the computer readable instructions are for controlling athrust unit or a stabilization system according to the second or thirdaspects.

The oscillating movement of the riser to be counteracted preferably hasa period of over 1 second. The oscillation also preferably has a periodof less than 30 minutes. In preferred embodiments, the oscillatingmotion has a period of between 3 seconds and 20 seconds, and preferablybetween 6 and 15 seconds. In other preferred embodiments, theoscillating movement has a period of between 1 minute and 5 minutes.

Preferably the oscillating movement of the riser to be counteracted isat a resonant frequency of the riser, and more preferably is at thelowest resonant frequent of the riser. The oscillating force may also beapplied to the riser so as to further counteract oscillating movement ofthe riser at a second, different frequency, which may be a secondresonant frequency of the riser.

Viewed from a fifth aspect, the present invention provides a computerprogram product comprising computer readable instructions that, whenexecuted, will cause a processor to perform a method comprising:receiving parameters of a riser; modelling oscillating movement of theriser when connecting a structure on the seabed to the sea surface; anddetermining a location for a thrust unit on the riser to counteract theoscillating movement based on the modelling. The determined location ispreferably an antinode of the oscillating movement and may be determinedby modelling as discussed above in relation to the methods of theinvention and preferred embodiments.

The location for the thrust unit should not be at an upper end or alower end of the riser, and preferably not within 10% of the length ofthe riser from the upper or lower ends of the riser.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain preferred embodiments of the invention will now be described ingreater detail, by way of example only, with reference to theaccompanying drawings, in which:

FIG. 1 shows a prior art offshore installation including a riser;

FIG. 2 shows an offshore installation including a riser stabilized by athrust unit;

FIG. 3 shows a side view of the thrust unit shown in FIG. 2;

FIG. 4 shows a plan view of the thrust unit shown in FIG. 2;

FIG. 5 shows a perspective view of an alternative thrust unit;

FIG. 6 shows a plan view of the thrust unit shown in FIG. 4; and

FIG. 7 shows an offshore installation including a riser stabilized by athrust unit.

DETAILED DESCRIPTION

FIG. 1 shows a simplified diagram of a conventional offshoreinstallation 10. The installation 10 comprises a floating platform 12(formally a floating production, storage, and offloading system), aproduction riser 14, and a wellhead 16. The production riser 14 connectsthe floating platform 12 to the wellhead 16 via a lower riser package 18(also known as a Christmas tree) and a blow-out protector (BOP) 20. Theriser 14 is typically tensioned by a heave compensating system, which isa system that tries to compensate for the relative movement of thefloating platform 12 with respect to the seabed, in order to improve thestability of the riser 14.

Fatigue problems at the upper part of the riser 14 occur at all waterdepths due to motion of the floating platform 12. It is predominantlyroll and pitch motions (i.e. rotation about the horizontal axes) thatcause bending moments in the riser 14, but surge and sway motions canalso contribute. Tension variations in a heave compensating system canalso lead to bending moment variations when the riser 12 is angled, forexample as in the case in FIG. 1. These combined effects result influctuating bending moments and bending stresses, causing fatigue damageto the upper part of the riser 14.

Fatigue problems at the lower part of the riser 14, as well as in thelower riser package 18 or the wellhead 16, occur most often at shallowwater depths, although fatigue problems in these regions can also occurat deep water depths when there are large tension variations in thesystem. A fluctuating bending moment at the bottom of the riser 14,which gives rise to fatigue stresses, is primarily induced by thefollowing two effects.

Firstly, the lower part of the riser 14 may be at an angle relative tothe wellhead 16 due to current forces, vessel offset between thewellhead 16 and the floating platform 12, a non-vertical wellhead 16,higher order oscillation of a floater due to higher order wind and wavedrift forces, as well as other factors. Where the lower part of theriser 14 is angled relative to the wellhead 16, tension variations, forexample due to the differential movement between the wellhead 16 and thefloating platform 12 cause fluctuating bending moments.

Secondly, horizontal oscillatory motions of the riser 14 caused by waveforces and wave motion of the floating platform 12 causes a varyingangle of the riser at the bottom. Eigenmodes, also known as resonantmodes, of the riser 14 can result in large dynamic amplification of thehorizontal riser motion.

Eigenmodes of the riser 14 at periods between 3 and 20 seconds, and morespecifically between 6 and 15 seconds are particularly dangerous. Thisis because this range corresponds to the typical frequency of oceanwaves acting on and exciting the floating platform 12. In deep waterdepths the drag of the water on the riser 14 provides significantdamping, moving the horizontal eigenmodes outside of this range, but inshallow water they can be a big problem. Thus, the effects of horizontaloscillatory motion due to ocean waves are amplified by shallow water,whilst in deeper water horizontal motion of the lower part of the riser14 is limited.

Furthermore, lower frequency oscillations of the riser 14, typicallyhaving a period of between 1 and 5 minutes, result from wind and wavedrift forces. These movements occur in both deep and shallow waters.

FIG. 2 shows an offshore installation 100 that has been stabilized inaccordance with the present invention. The installation 100 includes afloating platform 102 at the sea surface, a wellhead 104 on the seabed,and a riser 106 connecting the wellhead 104 to the platform 102. Mountedto the riser is a thrust unit 108.

The thrust unit 108 will be described in greater detail later, but iscapable of generating a thrust in any horizontal direction. The riser106 in this first embodiment is shown exhibiting oscillating movementhaving two nodes (at the platform 102 and the wellhead 104) and oneantinode (approximately at the middle of the riser 106).

The thrust unit 108 is mounted at a location along the length of theriser 106. The precise location is selected depending on a number offactors, as will be discussed in greater detail below, such as theparticular riser 106 properties, local weather conditions, number ofthrust units 108 used, etc. However, the location at which the thrustunit 108 is mounted is typically between 10% and 90% along the length ofthe riser 106 from the wellhead 104 to the platform 102, i.e. from theseabed to the sea surface.

A method of mounting the thrust unit 108 to the riser will now bediscussed.

A computer, such as the controller 118, stores a computer programproduct for modelling the behavior of the riser 106 and determiningwhere on the riser 106 to mount the thrust unit 108.

First, a user inputs parameters relating to the riser 106. These mayinclude, for example, the length of the riser 106, the stiffness of theriser 106, the type of platform 102, the end-fixing conditions of theriser 106, the weather and tidal patterns for the location where theriser 106 is to be installed (i.e. the location of the offshore platform102), etc. Many risers 106 are purchased “off-the-shelf”. Therefore,optionally, certain of the riser parameters may be pre-stored, either onthe computer or at a remote location, and the user may simply select alocation and/or model of the riser 106 from a list of known locationsand/or models.

Next, the computer models behavior of the riser 106. The computer may,for example, generate estimates of the forces to which the riser 106will be subjected based on the input parameters and, based on thoseforces, estimate the modes of oscillation of oscillating movement thatthe riser 106 might be expected to display.

The invention primarily relates to the cancellation of oscillatorymotion along the length of the riser 106. As such, the ends of riser 106may be treated as nodes of the waveform, as they remain relatively stillcompared with mid-points of the riser at the frequencies of interest.

Finally, the computer will determine the optimal location(s) on theriser 106 in which to mount a desired number of thrust units 108 so asto minimize the oscillation of the riser 106. This may, for example, beat the antinodes of the determined oscillating movement. The number ofthrust units 108 available may also affect the optimal locations. As theends of the riser remain relative stationary, the thrust unit will notbe located at an upper end or a lower end of the riser 106, and normallynot within 10% of the length of the riser 106 from the upper or lowerends of the riser 106.

Turning now to the details of thrust unit 108, an embodiment of a thrustunit 108 is shown in greater detail in FIGS. 3 and 4. The thrust unit108 comprises a sensor (not shown) and three pairs of axially offset(with respect to the axis of the riser 106) propeller-driven thrusters110, 112, 114. The thrusters are mounted to a frame 116 that is in turnreleasably mounted to the riser 106. The frame 116 is configured so asnot to move axially along the riser 106, nor to rotate about the riser106, when mounted thereto.

Each pair of thrusters 110, 112, 114 is controllable independently ofthe other pairs of thrusters 110, 112, 114, and may be controlled togenerate a forward or backward thrust along a respective thrustdirection. Each thrust direction passes through an axis of the riser 106so that no torque is generated by thrust along that thrust direction.

Each pair of thrusters 110, 112, 114 comprises a first thruster 110 a,112 a, 114 a and a second thruster 110 b, 112 b, 114 b that are arrangedrespectively on opposite sides of the riser 106. Each first thruster 110a, 112 a, 114 a is positioned with its direction of thrust parallel tothe direction of the corresponding second thruster 110 b, 112 b, 114 b.The direction of thrust of each thruster 110 a, 112 a, 114 a, 110 b, 112b, 114 b is substantially horizontal and each thruster 110 a, 112 a, 114a, 110 b, 112 b, 114 b can be operated in both a forward thrust and arearward thrust mode.

The pair of thrusters 110, 112, 114 are arranged such that the thrustdirection of each pair of thrusters 110, 112, 114 is non-parallel withthe thrust directions of the other two pairs of thrusters 110, 112, 114.In the present embodiment, the thrust directions of the pairs ofthruster 110, 112, 114 are oriented at equal intervals of 120°.

By selectively controlling the magnitude and directions of thrustgenerated by each of the pairs of thrusters 110, 112, 114, a resultingthrust force can be generated in any horizontal direction on the riser106. The thrust unit 108 has designed redundancy because only twonon-parallel thruster pairs are required to generate a resulting thrustin any horizontal direction. Thus, if any thruster pair 110, 112, 114fails, for example by one of the thrusters 110 a, 112 a, 114 a, 110 b,112 b, 114 b becoming ineffective or less effective, the thrust unit 108can still operate at maximum capacity.

The thrusutilized 08 is to be utilized to counter oscillating movementof the riser 106 occurring at the resonant modes of the riser 106.Typically, the greatest resonance occurs at the resonant mode having thelowest natural frequency (the fundamental mode). In order to maximizethe efficiency of the thrust applied by the thrust unit 106, and tominimize the inadvertent stimulation of other resonant modes, the thrustunit 108 is positioned at the antinode of the vibration. Whilst dragforces and end conditions of the riser 108 may cause the antinodeposition to move, the antinode of the fundamental mode of vibration istypically about midway along the length of the riser 106.

The sensor detects at least the horizontal velocity of the riser 106 atthe location of the sensor. The sensor may include an accelerometer fordetermining horizontal velocity. At the location of the sensor, thesensor may additionally detect the horizontal position of the riser 106,the acceleration of the riser 106, the relative speed of the watersurrounding the riser 106, the tension in the riser 106 and the bendingmoment in the riser 106.

The thrust unit 108 and sensor are connected to a controller 118. Thecontroller 118 is configured to receive inputs from the sensor and tocontrol the thrusters 110 a, 112 a, 114 a, 110 b, 112 b, 114 b of thethrust unit 108 to apply a force to the riser 106. The controller 118may be positioned either locally to the thrust unit 108, for examplemounted on the frame 116, or positioned elsewhere, for example on thefloating platform 100.

In use, the controller 118 controls the thrust unit 108 to provide anoscillating force to counteract oscillating movement of the riser 106detected by the sensor. In one example, this comprises applying a forceto the riser 106 proportional and opposite to the horizontal velocity ofthe riser 106.

Various implementations of how to counteract the oscillating movementbased on the inputs from the sensor and the model of the riser 106 willbe apparent to those skilled in the art. Further examples will thereforenot be set forth in length.

FIGS. 5 and 6 illustrate a thrust unit 208 having an alternativethruster configuration. The thrust unit 208 shown in FIGS. 5 and 6 issimilar to the thrust unit 108 shown in FIGS. 3 and 4 and like referencenumerals have been used to indicate like features. Description of thosefeatures common to both configurations and discussed previously has beenomitted.

The thrust unit 208 again comprises a sensor (not shown) and three pairsof thrusters 110, 112, 114. As with thrust unit 108, the first thruster110 a, 112 a, 114 a of each of the pairs of thrusters 110, 112, 114 ispositioned on an opposite side of the riser 106 to the respective secondthruster 110 b, 112 b, 114 b, and the first and second thrusters 110 a,112 a, 114 a, 110 b, 112 b, 114 b of each thruster pair 110, 112, 114are positioned with their direction of thrust parallel to one another

Different from thrust unit 108, each first thruster 110 a, 112 a, 114 ais provided in a first thrust plane, which is perpendicular to the riser106, and each second thruster 110 b, 112 b, 114 b is positioned in asecond thrust pane, which is also perpendicular to the riser 106 and isaxially offset with respect to the first thrust plane. However, thethrust direction of each pair of thrusters 110, 112, 114 still passesthrough an axis of the riser 106 so that no torque is generated bythrust along that thrust direction.

The first thrusters and second thrusters 110 a, 112 a, 114 a, 110 b, 112b, 114 b are mounted, respectively, to first and second mountingsurfaces of a frame 208. The frame 208 is in turn releasably mounted tothe riser 106 so as to prevent axial movement and relative rotation.

The first thrusters 110 a, 112 a, 114 a are arranged within the firstthrust plane such that their directions of thrust act are evenly spacedat 120° intervals. The second thrusters 110 b, 112 b, 114 b are oppositeto the first thrusters 110 a, 112 a, 114 a, and are thus similarlyarranged within the second thrust plane at evenly spaced 120° intervals.Thus, each plane may independently generate thrust in any horizontaldirection. As the thrusters are radially offset from the riser 106, thegenerated thrust in each plane may generate a twisting moment on theriser 106; however, this is counteracted by the thrust generated in theother thrust plane.

FIG. 7 shows another offshore installation 100 that has been stabilizedin accordance with the present invention. The installation 100 issimilar to the installation 100 described in the first embodiment andlike reference numerals have been used to indicate like features.Description of those features common to both embodiments and discussedpreviously has been omitted.

The installation 100 again includes the floating platform 102, thewellhead 104 on the seabed, and the riser 106 connecting the wellhead104 to the platform 102. Mounted to the riser are a first thrust unit120 and a second thrust unit 122. Both the first and second thrust units120, 122 are similar to thrust unit 108 described above; however, thrustunit 208 may also be used in this configuration.

The first and second thrust units 120, 122 are both connected to andcontrolled by the controller 118. Each of the first and second thrustunits 120, 122 includes a sensor (not shown) as discussed above,although only one sensor is required to determine the movement of theriser 106 as the movement of the rest of the riser can be determinedbased on the model. The sensors are also connected to the controller118.

The optimal locations for mounting the first and second thrust units120, 122 is determined by the method discussed above. The riser 106 inFIG. 7 is shown exhibiting oscillating movement having three nodes (atthe platform 102, the wellhead 104 and approximately at the middle ofthe riser 106) and two antinodes (approximately one quarter and threequarters along the length of the riser 106). As the dominant mode ofvibration has two antinodes, the first and second thrust units 120, 122are respectively positioned at the antinodes.

The controller 118 independently controls the first and second thrustunits based on the measurements from both sensors to counteractoscillating movement of the riser 108.

Whilst the invention has been described with reference to certainpreferred embodiments, it will be understood by those skilled in the artthat various other embodiments are also within the scope of theinvention.

For example, it will be understood that the described production riser106 of the floating platform 102 is merely an exemplary riser, and thatthe invention is applicable to any type of riser, such as export risers,production risers, drilling risers, and workover risers, utilized by anytype of offshore platform, such as fixed tower platforms, sparplatforms, tension-leg platforms, semi-submersible platforms, etc.

Additionally, whilst the described embodiment includes one sensor foreach thrust unit 108, 120, 122, 208, it will be apparent to thoseskilled in the art that a greater number of sensors may be positionedalong the riser 106, each being connected to the controller 118.Furthermore, a single sensor could be used to control two or more thrustunits 108, 120, 122, 208.

Also, whilst the described risers 106 have been shown exhibitingoscillations affecting the entire length of the riser 106, localizedoscillations may also occur affecting only a relatively short length ofthe riser 106. This may particularly be the case where othercounter-vibration measures are implemented.

Furthermore, whilst the described thrust units 108, 120, 122, 208 havebeen described as units formed separately of the riser 106, they may ofcourse be formed integrally with the riser, for example formed togetherwith a riser segment to be installed in the riser 106 duringmanufacture.

The invention is therefore not to be seen as being limited to thedescribed embodiment, but should be understood to include allembodiments falling within the scope of the claims.

1. A method of stabilizing a riser connecting a structure on the seabed to the sea surface, the method comprising: applying an oscillating force to the riser, using a thrust unit on the riser, so as to counteract an oscillating movement of the riser.
 2. A method according to claim 1, wherein the oscillating movement of the riser has a period of between 1 second and 30 minutes.
 3. A method according to claim 1, wherein the oscillating movement of the riser is at a resonant frequency of the riser.
 4. A method according to claim 1, further comprising: modelling the oscillating movement of the riser; determining a location for the thrust unit on the riser based on the modelling; and locating the thrust unit on the riser at the determined location.
 5. A method according to claim 4, wherein the determined location is based on a predicted location of an antinode of the modelled oscillating movement.
 6. A method according to claim 1, wherein the thrust unit comprises at least two pairs of thrusters arranged to provide thrust in non-parallel directions.
 7. A method according to claim 6, wherein the thrust unit comprises at least three pairs of thrusters, each being arranged to provide thrust in a non-parallel direction.
 8. A riser stabilization system for stabilizing a riser connecting a structure on the seabed to the sea surface, comprising: at least one sensor for detecting movement of the riser; a thrust unit for applying a force to the riser; and a controller configured to, in use, cause the thrust unit to apply an oscillating force to the riser so as to counteract an oscillating movement of the riser detected by the at least one sensor.
 9. A system according to claim 8, wherein the oscillating movement of the riser has a period of between 1 second and 30 minutes.
 10. A system according to claim 8, wherein the oscillating movement of the riser is at a resonant frequency of the riser.
 11. A system according claim 8, wherein the thrust unit comprises at least two pairs of thrusters arranged to provide thrust in non-parallel directions.
 12. A system according to claim 11, wherein the thrust unit comprises at least three pairs of thrusters, each being arranged to provide thrust in a non-parallel direction.
 13. A system according to claim 8, wherein the sensor is arranged to detect a horizontal velocity of the riser.
 14. A thrust unit for mounting to a riser, the thrust unit comprising: a frame configured to be mounted to the riser; and a plurality of pairs of thrusters, each thruster being mounted to the frame, wherein each pair of thrusters generates a net thrust in a respectively non-parallel direction.
 15. A thrust unit according to claim 14, wherein, for each pair of thrusters, a first thruster of the pair of thrusters is configured so as to be on an opposite side of the riser to a second thruster of the pair of thrusters, when the frame is mounted to the riser.
 16. A thrust unit according to claim 14, wherein the plurality of pairs of thrusters comprises three pairs of thrusters.
 17. A thrust unit according to claim 14, wherein the pairs of thrusters are mounted to the frame so that, when mounted to the riser, each of the pairs of thrusters are axially offset from the other pairs of thrusters.
 18. A thrust unit according to claim 14, wherein a first thruster of each pair of thrusters is mounted in a first thrust plane and a second thruster of each pair of thrusters is mounted in a second thrust plane that is axially offset along the axis of the riser, wherein the thrusters in each of the first and second thrust planes are configured to generate a net thrust in any direction within the thrust plane.
 19. A riser for connecting a structure on the seabed to the sea surface, comprising a system as claimed in claim 8 or a thrust unit comprising a frame configured to be mounted to the riser; and a plurality of pairs of thrusters, each thruster being mounted to the frame, wherein each pair of thrusters generates a net thrust in a respectively non-parallel direction, wherein the thrust unit is mounted on the riser.
 20. A computer program product comprising computer readable instructions that, when executed, will cause a processor to perform a method comprising: receiving indications of a movement of a riser connecting a structure on the seabed to the sea surface from at least one sensor; and causing a thrust unit to apply an oscillating force to the riser so as to counteract oscillating movement of the riser detected by the at least one sensor. 21.-23. (canceled) 